It is known that in industrial process control systems, specific field devices are widely used to detect/measure one or more physical variables of a process fluid, for example absolute, relative or differential pressure, flow, level and similar.
In one of the most widespread embodiments, said field devices comprise an appropriately configured shell which houses the various detection components, typically including a pressure sensor, hence the name pressure transmitters; the use of a pressure sensor as a transducer element allows the operator to easily obtain from one or more measurements of relative, differential or absolute pressure, measurement values relative to other physical variables of the process fluid controlled, for example level, which would otherwise be difficult to transduce directly. The transmitter shell also contains other components including primary electronic circuits for processing the signals coming from the pressure sensor; secondary electronic circuits for processing the signals coming from the primary electronic circuits which are then used for control of the communication with other transmitters or with control units; displays for in-situ display of the variables detected, terminal blocks for the various connections in addition to electric circuits for powering the components, etc.
To perform the detection and measurement operations required, each pressure transmitter is provided with one or more pressure-sensitive elements which interface with the process and are usually referred to by the term separator units, or separation diaphragms or membranes, or other equivalent terms.
In particular, the differential pressure transmitters are provided with two separator units, each of which comprises an elastic element, typically a flexible metallic membrane fixed on a support, which is usually also metallic; the separator unit is positioned on the instrument so that the membrane has an outer wall exposed to the process fluid and an inner wall hydraulically coupled to the pressure sensor, usually by means of a non-compressible fluid, for example silicone oil, contained in a connection capillary. In this way, the pressure exerted by the fluid on each of the flexible membranes causes the deformation thereof and is transferred by means of the hydraulic coupling fluid to the pressure sensor. In turn the sensor “senses” the pressure on the two branches of the instrument and transmits the corresponding signals to the electronic part for the various processing operations. Sensors currently used for these applications are usually of the capacitive, inductive or even piezoelectric type.
Measurements of this type are usually difficult as the accuracy required is very high vis-à-vis conditions of use that can negatively affect the measurements themselves, for example due to electromagnetic disturbances, changing ambient conditions—such as temperature variations—or intrinsically difficult operating conditions such as potentially explosive environments.